The present invention relates to a method for allocating channels in a radio communications system; in particular, in mobile radio systems designated as the UMTS (Universal Mobile Telecommunications System) with a broadband radio interface. The present invention also relates to a base station system, a subscriber station and a radio communications system for implementing the method according to the present invention.
In radio communications systems, data is transferred using electromagnetic waves via a radio interface. The radio interface relates to a connection between a base station and subscriber stations, whereby the subscriber stations can be mobile stations or fixed radio stations. The electromagnetic waves are thereby emitted with carrier frequencies, which are located within the frequency band provided for the system in question. For future radio communications systems, such as the UMTS or other 3rd generation systems, frequencies in the frequency band of approximately 2000 MHz are provided, whereby the bandwidth of a channel is 5 MHz.
Unlike systems such as the GSM (Global System for Mobile communications), a number of services that also have to be transmitted in parallel are provided for in the UMTS. In patent specifications EP 98 122 719 and DE 198 55 194, possibilities are disclosed for signaling the transport formats of the combination of data of a number of services. The data of a number of services of one connection is thereby transferred via a mutually-used physical channel.
The use of mutually-used physical channels for the transfer of data of a number of services of a connection to a subscriber station presupposes that a unique mapping specification sets out the assignment of the services to different segments of the physical channel. A physical channel is, for example, defined by a frequency band and a spread code (CDMA Code Division Multiple Access) within a frame.
The following terms are generally used to describe the mapping specification:
Transport Format (TF):
A transport format defines a data rate, coding, interleaving, data rate adjustment by dotting and an error protection specification of a transport channel for a service.
Transport Format Set (TFS):
This designates a set of possible transport formats, which are permitted for a specific service.
Transport Format Combination (TFC):
This term specifies a possible combination of transport formats of the different services, which are mapped onto a shared physical channel.
Transport Format Combination Set (TFCS):
This designates a set of possible TFCs as a subgroup of all TFCs, which are permitted for a specific connection.
Transport Format Combination Identifier (TFCI):
This information specifies the combination of transport formats in current use within the TFCS.
In order for it to be possible to select the currently used combination of transport formats of the different services as required, it is necessary to be able to modify the TFC and, therefore, to signal the TFCI in a regular manner. However, such signaling disadvantageously ties up transfer capacity. The larger the number of possible combination options (TFCS) the more capacity is required for signaling.
With the broadband CDMA system selected for the FDD (Frequency Division Duplex) mode of the UMTS, the problem occurs during transmission from the base station to the subscriber station in the downward direction (downlink) that the number of simultaneously useable orthogonal spread codes is limited and the support of variable data rates is thereby hampered. It is, therefore, not possible with higher traffic densities in the system to assign as many dedicated channels (DCH) (i.e., those used exclusively by the subscriber station), to all subscriber stations, as these might require for transfer at their maximum respective data rates.
For this reason, what are known as shared channels are defined in the downward direction (DSCH Downlink Shared Channel); see also, ETSI SMG2 UMTS-L1, Tdoc SMG2 UMTS-L1 559/98, dated 9 Nov. 1998. The shared channels are created within the broadband frequency band via spread codes, which are temporarily assigned to different connections or subscriber stations for the period of one or a number of frames, respectively. However, the problem then arises of how it can be signaled to a subscriber station with minimum outlay whether and if so in which of these shared channels information is transferred for the subscriber station.
It is also known from ETSI SMG2 UMTS-L1, Tdoc SMG2 UMTS-L1 559/98, dated 9 Nov. 1998, that the data rates for the services transmitted in the time multiplex are signaled via the TFCI parameter, which is transferred during each frame as part of the control information; i.e., in-band. To ensure the fast allocation of shared channels, an explicit signaling is proposed which uses a specific number of TFCI bits solely to display a specific spread code (see penultimate page of the cited document).
This solution has the disadvantage that for a given number of TFCI bits the number of combination options for service transport formats is significantly restricted, which has an adverse effect on flexibility for the transfer of variable data rates.
It is also known in this context from DE 198 56 834 A1 that the shared channels used are signaled implicitly via the data rate and that a number of combinations of channels is permitted as an alternative only with specific data rates of the individual services.
In the case of a dedicated connection, it is established which resources are available at the start of the connection. As the data rate generally is not constant over the entire duration of the connection, with the known methods the current data rate is signaled separately for each allocation period. With the UMTS, the allocation period corresponds, for example, to what is known as the TTI (Transmission Time Interval). The subscriber derives the resources currently in use from such data rate.
Example: The data rate of the connection is established at 100 kB maximum and the resources 0-19 are, therefore, allocated to the subscriber. If the data rate then drops briefly to 50 kB, only the resources 0-9 are used. The remaining resources remain unused for this period.
In the case of mutually used resources, it must be established for each transfer which subscriber is permitted to use which resources. There are a number of possibilities here:                1. Only one subscriber can use all the resources in each instance. Subscribers would, therefore, be able to access the resources only according to a time-multiplex method. If a subscriber receives only a small quantity of data, the remaining resources would be unused.        2. Each subscriber is specifically informed which resources they are allowed to use. Example: Subscriber (TN) 1 uses resource 0-3, TN 2 resource 4 and 8, TN 3 resource 5 and 12-36. This method offers a high level of flexibility but has the disadvantage that a large number of bits have to be signaled.        3. There are also various gradations between the methods for possibilities 1 and 2.        
With all the methods disclosed, the resource information is forwarded specifically to the subscribers. The current data rate is also transferred in parallel to each subscriber. This disadvantageously results in a large signaling load.
Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the Figures.